1. Field of the Invention
The invention relates to an analytical method for hydrogen sulfide and a kit for carrying out the method.
2. Description of the Prior Art
For various reasons, it is frequently desirable to know the amount of hydrogen sulfide present in a sample of liquid, and for this purpose, many laboratory techniques have been developed. See for example Marsh, U.S. Pat. No. 3,660,035, which teaches dispersing the H.sub.2 S in a known quantity of a hydrocarbon into an ionic state and titrating with a standard cadmium salt solution to determine the hydrogen sulfide content of the hydrocarbon. Such laboratory techniques, however, are not easily adapted for field use, particularly by personnel who are untrained in analytical techniques. Various colorimetric H.sub.2 S test kits are available, such as the Hach Model HS-6 and HS-7 kits (illustrated in Hach Catalog no. 11A, page 27), for field analysis of aqueous samples. These kits, however, are said to be useful for water samples, but use on oils or emulsions is not suggested. Also, quantitative measure ment is based on a visual evaluation of the degree or intensity of color change, so that results are not particularly accurate. Withair, Chemistry and Industry, Apr. 19, 1975, page 355 discloses that a portable chromatographic method and apparatus is used for measuring the amount of H.sub.2 S in crude oil, but does not disclose the nature of the proprietary detector tube essential to practice the method. Moreover, the system of Withair is not useful on aqueous samples.
In some instances, it is necessary to know whether or not a sample contains H.sub.2 S in excess of a certain preselected concentration. For example, in oil field operations in Texas, it is necessary to now whether fluids employed in well treatments have in excess of 100 milligrams per liter (mg/l) H.sub.2 S to determine whether the provisions of Texas Railroad Commission Rule 36 apply. Since produced formation fluids--both aqueous, oil based, and emulsified fluids--are frequently reinjected as treating fluids, a single method is needed to quickly give an accurate analysis as to whether the preselected H.sub.2 S concentration is exceeded, regardless of whether the sample is oil based, water based, or emulsified. Although the method must give an accurate indication as to whether the preselected concentration is exceeded, the method need not quantitatively indicate by how much the preselected concentration is exceeded or by how much the preselected concentration exceeds the actual concentration.
Also of interest is Encyclopedia of Industrial Analysis, Vol. 15, page 568, which teaches in conjunction with analyzing for mercaptans in hydrocarbons, that H.sub.2 S should first be removed from the sample by precipitation using a solution of cadmium sulfate in sulfuric acid.
The teachings of each of the aforementioned patents are expressly incorporated herein.